1. Field of the Invention
The present invention relates to an electrophotographic apparatus for controlling the surface potential of a photosensitive member, and in particular relates to an electrophotographic apparatus that can select one of multiple target potentials as the surface potential for a photosensitive member.
2. Related Background Art
A widely employed technique for setting a desired dark portion potential and a desired light portion potential for an eloectrophotographic photosensitive member, involves the use of detection means, called a potential sensor, for measuring dark portion potential and light portion potential.
Whenever it is necessary to form a stable dark portion potential and a stable light portion potential, the potential control is initiated either at the time of or after a predetermined time has elapsed following the activation of an image forming apparatus, before the first output or after a predetermined time has elapsed following it, or upon the receipt of an artificial execution command.
As a popular sequence for obtaining a desired dark portion potential and a desired light portion potential, since the exposure amount of the exposure means is substantially xe2x80x9c0xe2x80x9d, first, the charge amount of the electrification means is adjusted so as to set a desired dark portion potential, and then, the exposure amount (quantity) of the exposure means is determined by using the charge amount (quantity) whereat the dark portion potential was obtained.
The relationship between the exposure amount of the photosensitive member used for electrophotography and the surface potential thereof seldom is a linear relationship to the exposure amount, and generally, the inclination is reduced when the exposure amount is large. In this case, even when the exposure amount is changed, a change of the dark portion potential can not be precisely read, and the measurement error is increased.
As a countermeasure for this phenomenon, two types of light portion potentials are employed. To obtain the first light portion potential, an exposure amount E1 is adjusted in a region wherein the inclination of the potential to the exposure amount is comparatively large, and either a specific value is added to the exposure amount or the exposure amount is multiplied by a specific rate, so that the final desired light portion potential is obtained as the second light portion potential. Thus, an image forming apparatus is generally employed that uses the second light portion potential to output an image. That is, the first light portion potential is detected by a sensor for the potential adjustment, and the second light portion potential is used to actually form an image.
However, according to the conventional potential control method using the two light portion potentials, when an image forming apparatus employing multiple dark portion potentials has (stores) only one value to be added to the first exposure amount, or only one ratio relative to the first exposure amount, the second light portion potential, that is, the final light portion potential, is greatly changed.
For various reasons, multiple different dark portion potentials are employed to change the dark portion potential, e.g., because of an image forming difference between a copy image and a printer image, because of a reduction in the amount of toner consumed, because of the need to prevent the leakage of electrification means in a low atmosphere, because of the need to adjust an image density, or because of development characteristics due to an environment.
Since even each dark portion potential has several types or a great number of levels, there could be several tens to an infinite number of combinations including the dark portion potential and the light portion potential that corresponds to the individual conditions.
On the other hand, in order to constantly obtain the same light portion potential, several tens to an infinite number of combinations must be stored for either the rate relative to the first exposure amount or the value to be added to the exposure amount. Thus, a load for the preparation for the process sequence or a load for a hard configuration, such as a memory, becomes very large.
It is one objective of the present invention to provide an electrophotographic apparatus that can appropriately determine the potential of a photosensitive member.
It is another objective of the present invention to provide an electrophotographic apparatus that can appropriately determine a light portion potential, regardless of the different dark potentials that may have been set for a photosensitive member.
It is an additional objective of the present invention to provide an electrophotographic apparatus that can determine many light portion potentials that correspond to different dark portion potentials on a photosensitive member.